Inverter device and method of manufacturing the device thereof, and electric automobile incorporating the inverter device thereof

ABSTRACT

By providing a plurality of semiconductor chips that are connected in parallel and constitute one arm of an inverter; a first conductor to which a face on one side of said plurality of semiconductor chips is connected; a wide conductor to which a face on the other side of said plurality of semiconductor chips is connected; a second conductor connected to said wide conductor; and a cooler to which said first conductor and second conductor are connected through an insulating resin sheet, part of the heat loss generated in the semiconductor chips is thermally conducted to the first conductor and is thence thermally conducted to the cooler, producing cooling, while another part thereof is thermally conducted to the wide conductor and thence to the second conductor, whence it is thermally conducted to the cooler, producing cooling.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims benefit of priority to Japanese Application No.JP 2003-321462 filed 12th September, 2003, the entire content of whichis incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an inverter device of high reliabilitywhich is of small size and excellent cooling efficiency, and a method ofmanufacturing the inverter device thereof and an electric automobile(sometimes called an electric vehicle or an electric car) incorporatingthe inverter device thereof.

2. Description of the Related Art

Miniaturization and improvement of reliability of power semiconductorelements and inverter devices employing these are being demanded. Whensemiconductor elements and inverter devices employing these are employedin an electric automobile, reduction in size and improved reliabilityare particularly important.

In order to achieve reduced size and improved reliability of powersemiconductor elements and inverter devices employing these, powersemiconductor elements and inverter devices are required that haveexcellent cooling efficiency such as Japanese Laid-Open patent2003-153554.

The construction of a conventional inverter device is described belowwith reference to FIG. 1 and FIG. 2. FIG. 1 is a partial axialcross-sectional view showing the mounting structure of semiconductorchips in the interior of a power semiconductor element; FIG. 2 is apartial perspective view of a power semiconductor element.

In the inverter device shown in FIG. 2, a single arm of a three-phaseinverter is constituted by connecting in parallel a plurality of IGBTs(insulated gate bipolar transistors) 171 and diodes 181 constituted bysemiconductor chips of size no more than 10 mm square; furthermore, aplurality of semiconductor chips constituting a single arm of thethree-phase inverter are bonded with a conductor 20 of thickness between1.5 mm and 5 mm; the conductor 20 is adhesively fixed to a cooler 22 bya ceramics-containing insulating resin sheet 23. (“10 mm square” or “□10mm” indicates that the length of one side of a square shape is 10 mm.)

Furthermore, in the inverter device shown in FIG. 2, fourparallel-connected IGBTs 171A to 171D constituting an upper arm of the Wphase of a three-phase inverter and two parallel-connected diodes 181Ato 181B are arranged in a single row with an upper arm conductor 25constituting an upper arm of the three-phase inverter; likewise, fourparallel-connected IGBTs 172A to 172D constituting a lower arm of the Wphase of a three-phase inverter and two parallel-connected diodes 182Ato 182B are arranged in a single row with a lower arm conductor 26constituting a lower arm of a three-phase inverter. In addition, athree-phase output conductor 27 that connects three-phase outputterminal 32 with the IGBTs 171A to 171D and the diodes 181A to 181B isarranged on the upper arm conductor 25 between the upper arm conductor25 and the lower arm conductor 26.

The construction of the upper arms and the lower arms will now bedescribed. The arms are as follows in the case of an inverter devicethat converts DC current to three-phase AC current as shown in FIG. 3.Specifically, in order to generate three-phase AC power from DC batterypower, an inverter is constituted comprising three-phase arms (U phase,X phase), (V phase, Y phase) and (W phase, Z phase) and conversion to ACcurrent is effected therein. This AC current provides three-phase ACpower for driving a three-phase motor.

The upper arms are constituted by the U phase, V phase and W phase andthe lower arms are constituted by the X phase, Y phase and Z phase.

In FIG. 2, the lower arm conductor 26 and the three-phase outputconductor 27 are constituted by the same conductor. In addition, anegative electrode conductor 28 that connects the negative electrodeterminal 31 with the IGBTs 172A to 172D and the diodes 182A to 182Barranged on the lower arm conductor 26 is arranged between the upper armconductor 25 and the lower arm conductor 26. Electrical connection iseffected by means of wire bonding 29 between the respective conductorsand the IGBTs and diodes.

In the inverter device shown in FIG. 1 to FIG. 2 the IGBTs 171A to 171Dand the diodes 181A to 181C bonded with the conductor 20 and/or upperarm conductor 25 and lower arm conductor 26 are directly connected witha cooler 22 using insulating resin sheet 23, so the thermal resistanceof the IGBTs and diode chips within the interior of the powersemiconductor element is reduced. Furthermore, since the IGBTs 171A to171D and diodes 181A to 181C are bonded with the conductor 20 and/orupper arm conductor 25 and lower arm conductor 26 of thickness between1.5 mm and 5 mm, the thermal time constant becomes large, due to theeffect of the thermal capacity of the conductor 20 and/or upper armconductor 25 and lower arm conductor 26, with the result that transientthermal resistance become small and the rise in temperature duringstart-up of the inverter becomes small. Cooling efficiency is thereforeimproved and miniaturization of the inverter device can be achieved.

However, with the conventional inverter device, although the thermalresistance of the IGBTs and the diode chips in the interior of the powersemiconductor elements was reduced, there were the following problems.

First of all, manufacturing time was prolonged since considerable timewas required to provide the plurality of wire bonding locations involvedin the electrical wiring of the plurality of parallel-connected IGBTs ordiode chips by wire bonding.

Also, there were structural limits to the extent to which the coolingefficiency could be improved.

Accordingly, one object of the present invention is to provide a novelinverter device and method of manufacturing the device thereof and anelectric automobile incorporating this inverter device wherein powersemiconductor elements, that provide excellent manufacturingcharacteristics, are employed, in order to improve current capacity andreduce the size of the inverter device, but wherein the coolingefficiency of the power semiconductor elements is further improved.

SUMMARY OF THE INVENTION

In order to achieve the above object, an inverter device according tothe present invention is constructed as follows. Specifically, aninverter device according to the present invention comprises: aplurality of parallel-connected semiconductor chips constituting an armof an inverter;

-   -   a first conductor to which one face of this plurality of        semiconductor chips is connected;    -   a wide conductor to which the other face of this plurality of        semiconductor chips is connected;    -   a second conductor connected to the wide conductor; and    -   a cooler to which said first conductor and second conductor are        adhesively fixed, with an insulating resin sheet interposed        therebetween.

According to the present invention, the current capacity of the inverterdevice can be improved and reduction in size and improved reliabilityalso achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a partial vertical cross-sectional view showing the mountingstructure of semiconductor chips within the interior of a conventionalpower semiconductor element;

FIG. 2 is a partial perspective view of a conventional powersemiconductor element;

FIG. 3 is a view given in explanation of the construction of an arm;

FIG. 4 is a partial perspective view showing the mounting structure ofan inverter device according to a first embodiment of the presentinvention;

FIG. 5 is a view showing the results of analysis of heat flux showingthe heat discharge paths of loss of heat generated by a semiconductorchip during passage of current by the inverter device of FIG. 4;

FIG. 6 is a view showing the results of analysis of transient thermalresistance of a semiconductor chip of the inverter device of FIG. 4;

FIG. 7 is a partial perspective view showing the mounting structure ofan inverter device according to a second embodiment of the presentinvention;

FIG. 8A, FIG. 8B and FIG. 8C are respectively partial perspective viewsshowing the sequence of manufacturing an inverter device according tothe second embodiment of the present invention;

FIG. 9 is a simplified transparent perspective view of the case where aninverter device according to the present invention is incorporated in anelectric automobile; and

FIG. 10 is a simplified transparent perspective view of the case wherean inverter device according to the present invention is incorporated inan electric automobile that also incorporates an internal combustionengine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, and moreparticularly to FIG. 4 thereof, one embodiment of the present inventionwill be described.

FIG. 4 is a partial perspective view showing the mounting structure ofan inverter device according to a first embodiment of the presentinvention. FIG. 5 is a view showing the results of analysis of heat fluxshowing the heat discharge paths of loss of heat generated by asemiconductor chip during passage of current by the inverter device ofFIG. 4. FIG. 6 is a view showing the results of analysis of transientthermal resistance of a semiconductor chip of the inverter device ofFIG. 4.

In the inverter device of FIG. 4, one arm of the three-phase inverter isa constituted by connecting in parallel IGBTs 171A to D and diodes 181Ato B and IGBTs 172A to D and diodes 182A to B, which are semiconductorchips of size less than 10 mm square (“10 mm square” indicates that thelength of one side of a square shape is 10 mm.). FIG. 4 shows the casewhere four IGBTs are connected in parallel and two diodes are connectedin parallel.

Furthermore, four parallel-connected IGBTs 171A to 171D constituting anupper arm of a three-phase inverter and two parallel-connected diodes181A to 181B are arranged in a single row with an upper arm conductor 25constituting an upper arm of a three-phase inverter; likewise, fourparallel-connected IGBTs 172A to 172D constituting a lower arm of thethree-phase inverter and two parallel-connected diodes 182A to 182B arearranged in a single row with a lower arm conductor 26 constituting alower arm of the three-phase inverter.

In addition, three-phase output conductors 27 that connect three-phaseoutput terminals 32 with the IGBTs 171A to 171D and the diodes 181A to181B are arranged on the upper arm conductor 25 between the upper armconductor 25 and the lower arm conductor 26. In the embodiment of FIG.4, the lower arm conductor 26 and the three-phase output conductor 27are constituted by the same conductor.

In addition, a negative electrode conductor 28 that connects thenegative electrode terminal 31 with the IGBTs 172A to 172D and thediodes 182A to 182B arranged on the lower arm conductor 26 is arrangedbetween the upper arm conductor 25 and the lower arm conductor 26.

The thickness of the upper arm conductor 25 and lower arm conductor 26and three-phase output conductor 27 and negative electrode conductor 28is about 1.5 mm to 5 mm. The IGBTs and the diodes are arranged dispersedby distances of at least twice the thickness of the aforesaid conductorsand are bonded with the conductors by low melting point solder such asSn/Pb or high melting point solder such as Sn/Ag/Cu. The inventors ofthe present invention believe that the latter i.e. high melting pointsolder will often be used in future in view of environmental engineeringaspects.

In addition, the upper arm conductor 25 and lower arm conductor 26 andthree-phase output conductor 27 and negative electrode conductor 28 areadhesively fixed to the cooler 22 by means of a ceramics-containinginsulating resin sheet 23. The insulating resin sheet 23 is filled withceramic filler such as for example boron nitride in the insulating resinand has a thermal conductivity of 2 to 4 W/mK and a thickness of about0.05 to 0.15 mm.

In addition, a wide conductor 33 is bonded with the upper parts of theIGBTs and diodes by means of low melting point solder such as Sn/Pb orhigh melting point solder such as Sn/Ag/Cu, so that these areelectrically connected by means of the three-phase output conductor 27and negative electrode conductor 28 and wide conductor 33.

The upper arm conductor 25 and positive electrode terminal 30, and thethree-phase output conductor 27 and three-phase output terminal 32, andthe negative electrode conductor 28 and negative electrode terminal 31are electrically connected by means of an input/output conductor 34.

Since the material of the IGBTs 171 and IGBTs 172 and of the diodes 181and diodes 182 is silicon (Si), while the material of the variousconductors is copper (Cu), the linear expansion coefficient of the IGBTsand diodes and the conductors differ. As a result, when loaded by thetemperature cycle, shearing stress is generated in the solder bondingthe IGBTs and diodes with the conductors, resulting in the production ofnon-linear strain. If the value of the non-linear strain becomes large,cracking etc is generated in the solder when loaded by the temperaturecycle. Reduction of the value of the non-linear strain improvesreliability and durability.

Consequently, when the chip size of the IGBTs and diodes is large,non-linear strain of the solder is increased. It is therefore necessaryto keep the chip size below about 10 mm square in order to ensurereliability and durability of the inverter device.

However, if the chip size is less than 10 mm square, the currentcapacity per chip is small, so in order to construct an inverter deviceof a few tens of kW for use in an electric automobile, chips must beconnected in parallel.

Also, in order to reduce the rise in temperature in particular onstart-up of the inverter and in order to reduce the steady thermalresistance by the heat dispersion effect of the conductors, thethickness of the upper arm conductor 25 and lower arm conductor 26 andthree-phase output conductor 27 and negative electrode conductor 28 ispreferably 1.5 to 5 mm.

A three-phase RST AC power source is only produced by providing threesuch inverters (elements) having three-phase output terminals 32.

FIG. 5 shows the results of analysis of heat flux in a steady condition,showing the flow of heat loss when cooling is effected by thermalconduction of heat loss generated in an IGBT 172A constituted by asemiconductor chip to the cooler 22 during passage of current by theinverter device. The arrows show the direction of the heat flux and themagnitude of the arrows shows the magnitude of the heat flux.

FIG. 6 shows the results of analysis comparing the transient thermalresistance of the prior art power semiconductor element shown in FIG. 1and FIG. 2 and a semiconductor chip of a power semiconductor elementaccording to the present invention.

In the prior art power semiconductor element, the wire bonding isextremely fine, so the thermal resistance is extremely large.Consequently, as shown in FIG. 5, practically all of the heat lossgenerated in the IGBT 172A is thermally conducted to the lower armconductor 26 and emitted to the cooler 22.

In contrast, as shown in FIG. 5, in the case of a power semiconductorelement according to the present invention, part of the heat lossgenerated in the IGBT 172A is thermally conducted to the wide conductor33 and thence thermally conducted to the negative electrode conductor28, while another part thereof is thermally conducted to the lower armconductor 26 and is thence conducted to the cooler 22, thereby achievingcooling.

In this analysis, it is assumed that the thickness of the wide conductor33 is 3 mm, and the thickness of the lower conductor 26 and negativeelectrode conductor 28 is 3 mm. In order to promote thermal conductionfrom the wide conductor 33, since large cross-sectional area impliessmall thermal resistance, preferably the wide conductor should have somedegree of thickness (of the order of a few mm). Although the thermalresistance can be reduced by making the cross-sectional area large, theproblem of increased weight is generated.

As described above, according to the present invention, both faces ofthe IGBT 172A are cooled, by means of the wide conductor 33 and thelower arm conductor 26, so, as shown in FIG. 6, the steady thermalresistance after 10 to 20 sec is lowered by about 25% compared with thatof a conventional power semiconductor element. The transient thermalresistance for 0.1 to 0.3 sec on start-up of the inverter, which used topresent a problem, is also reduced by about 50% compared with that ofthe prior art power semiconductor element, due to cooling of both sidesof the wide conductor 33 and the thermal capacity effect.

Also, if, in a power semiconductor element according to the presentinvention, the electrical wiring is completed by bonding the IGBTs anddiodes and conductors and wide conductor using solder, the steps ofwiring a plurality of wire bonding locations, as in the prior art powersemiconductor element, become unnecessary, reducing the manufacturingtime.

In this way, in an inverter device according to the first embodiment,the thermal resistance of the IGBTs and diode chips in the interior ofthe power semiconductor element is further reduced, both on inverterstart-up and in the steady condition, thereby lowering the rise intemperature of the IGBTs and diode chips and improving coolingefficiency. In this way, improvement in reliability and size reductionof the inverter device can be achieved and the manufacturing yield(manufacture yield) of the power semiconductor element can also beimproved.

Next, a second embodiment of the present invention will be described.

FIG. 7 is a partial perspective view showing the mounting structure ofan inverter device according to a second embodiment of the presentinvention. FIG. 8A, FIG. 8B and FIG. 8C are partial perspective viewsshowing the manufacturing sequence of an inverter device according tothe second embodiment of the present invention.

In FIG. 7, heat buffer plates 35 are bonded by low melting point soldersuch as Sn/Pb or high melting point solder such as Sn/Ag/Cu with theupper parts of the semiconductor chips constituting IGBTs 171A to D anddiodes 181A to B and IGBTs 172A to D and diodes 182A to B of the powersemiconductor element and the wide conductor 33 is adhesively fixed byelectrically conductive adhesive to the top of the heat buffer plates35.

The material of the heat buffer plates 35 is for example molybdenum (Mo)or the like, which has a linear expansion coefficient close to that ofthe semiconductor chips and its thickness is about 0.5 mm. Theelectrically conductive adhesive has a heat conductivity of about 15 to60 W/mK and is for example an adhesive whose matrix consists ofinsulating resin but which is filled with a conductive substance such assilver filler. Other details concerning the construction are the same asin the first embodiment.

In the above construction, heat buffer plates 35 and conductive adhesiveare interposed between the semiconductor chips and the wide conductor33, but, if for example the heat buffer plates 35 are made ofmolybdenum, with a thickness of 0.5 mm and the thermal conductivity ofthe electrically conductive adhesive is 15 W/mK, albeit the thermalresistance is about 5% greater than that of the first embodiment, thesteady resistance after 10 to 20 sec is reduced by about 20% comparedwith the prior art power semiconductor element.

Furthermore, as shown in FIG. 8B, first of all, the IGBTs 171A to D anddiodes 181A to B and IGBTs 172A to D and diodes 182A to B and heatbuffer plates 35 and the aforesaid conductors of the power semiconductorelement according to the present invention are bonded by low meltingpoint or high melting point solder.

After this, adhesive fixing by heating of the conductors and the cooler22 using insulating resin sheet 23 and, in addition, adhesive fixing byheating of the heat buffer plates 35 and the wide conductor 33 iseffected using electrically conductive adhesive, in respective steps atthe same temperature.

Furthermore, since the wide conductor 33 is divided for eachsemiconductor chip, adhesive fixing using pressure and heat is effectedusing insulating resin sheet 23 by applying pressure to the conductorsand cooler 22 at the locations where a plurality of semiconductor chipsare not bonded on each conductor.

With a power semiconductor element manufactured as described above,adhesive fixing with the cooler 22 by pressure and heating can beachieved using the insulating resin sheet 23 in steps at the sametemperature with the respective conductors in an independent condition,but in a condition in which the conductors are not integrated with thewide conductors 33 by solder bonding, as they are in the firstembodiment, even when variations occur in the degree of planarity orwarping of the adhesive fixing surfaces of the conductors with respectto the cooler, in the case of upper arm conductors 25 and lower armconductors 26 and the three-phase output conductors 27 and negativeelectrode conductors 28 wherein the output capacity of the inverter islarge, increasing the number of semiconductor chips arranged in paralleland increasing the length of the conductors. Adhesive fixing cantherefore be achieved in a condition in which the cooler 22 and theconductors are totally integrated.

Thus, in this second embodiment, even if the output capacity of theinverter device is increased, the manufacturing yield of the powersemiconductor elements can be improved and, both during inverterstart-up and in the steady condition, the temperature rise of the IGBTsand diode chips is low, being practically equivalent to that of thefirst embodiment, and the cooling effect is improved.

FIG. 9 is a diagrammatic transparent perspective view showing the casein which an inverter device as described above is incorporated in anelectric automobile. The inverter device 91 as described above isincorporated in the interior of the front part of the vehicle body 90 ofthe electric automobile, and a three-phase electric motor 92 that issupplied with power by this inverter device is provided in the vehiclebody 90. Drive wheels 93 are driven by this three-phase electric motor92, supplying propulsive force to the electric automobile.

FIG. 10 is a diagrammatic transparent perspective view showing the casein which an inverter device as described above is incorporated in anelectric automobile. The inverter device 101 as described above isincorporated in the interior of the front part of the vehicle body 100of the electric automobile, and a three-phase electric motor 102 that issupplied with power by this inverter device is provided in the vehiclebody 100. Drive wheels 103 are driven by this three-phase electric motor102, supplying propulsive force to the electric automobile.

This vehicle body 100 also incorporates an internal combustion engine104 and the drive wheels 103 are also driven by this internal combustionengine 104. Such an electric automobile is termed a so-called hybridvehicle, in which the drive wheels 103 are driven as required by thethree-phase electric motor 102 and/or the internal combustion engine104.

Although the above description was in respect of an inverter deviceemployed in an electric automobile, the present invention is notrestricted to this and could of course be applied to other applications.The present invention can usefully be applied in particular to elevatorsor electric trains.

Furthermore according to the above description IGBT is used as asemiconductor chip, however, other semiconductor chips such as MOSFET(Metal Oxide Semiconductor Field Effect Transistor) and the like may beused with the same result. Not only Si but also SiC with a wide gapsemiconductor as material of the semiconductor chip may be used with thesame result.

1. An inverter device, comprising: a plurality of semiconductor chipsthat are connected in parallel and constitute an arm of an inverter; afirst conductor to which a face on one side of said plurality ofsemiconductor chips is connected; a wide conductor to which a face onthe other side of said plurality of semiconductor chips is connected; asecond conductor connected to said wide conductor; and a cooler to whichsaid first conductor and second conductor are connected through aninsulating resin sheet.
 2. An inverter device, comprising: a firstsemiconductor chip group wherein a plurality of semiconductor chipsconstituting an upper arm of an inverter are connected in parallel; afirst conductor to which faces on one side of said semiconductor chipsof said first semiconductor chip group are connected; a secondsemiconductor chip group wherein a plurality of semiconductor chipsconstituting a lower arm of said inverter are connected in parallel; asecond conductor to which faces on one side of said semiconductor chipsof said second semiconductor chip group are connected; a first wideconductor to which faces on the other side of the semiconductor chips ofsaid first semiconductor chip group are connected; a second wideconductor to which faces on the other side of said semiconductor chipsof said second semiconductor chip group are connected; a third conductorconnected with a three-phase output electrode connected to said firstwide conductor and arranged between said first conductor and secondconductor; a fourth conductor connected with a negative electrodeconnected with said second wide conductor and arranged between saidfirst conductor and second conductor; and a cooler to which said firstto fourth conductors are connected through an insulating resin sheet. 3.The inverter device according to claim 1 or 2, further comprising: aheat buffer plate connected to said wide conductor at a face on theother side of said semiconductor chip.
 4. A method of manufacturing aninverter device, comprising: bonding a plurality of semiconductor chipsand a heat buffer plate by a low melting point or high melting pointsolder; bonding said heat buffer plate and a conductor by a low meltingpoint or high melting point solder; fixing said conductor to a cooler;and fixing said heat buffer plate and a wide conductor.
 5. The method ofmanufacturing an inverter device according to claim 4, furthercomprising: dividing said wide conductor into a plurality of chips; andfixing said conductor and said cooler by application of pressure atlocations where a plurality of semiconductor chips are not bonded tosaid conductor.
 6. An electric automobile, comprising: (1) an inverterdevice comprising: a plurality of semiconductor chips that are connectedin parallel and constitute an arm of an inverter; a first conductor towhich a face on one side of said plurality of semiconductor chips isconnected; a wide conductor to which a face on the other side of saidplurality of semiconductor chips is connected; a second conductorconnected to said wide conductor; a cooler to which said first conductorand second conductor are connected through an insulating resin sheet;and (2) an electric motor incorporating said inverter device and thatdrives a drive wheel by using an AC power from said inverter device. 7.An electric automobile, comprising: (1) an inverter device comprising: aplurality of semiconductor chips that are connected in parallel andconstitute an arm of an inverter; a first conductor to which a face onone side of said plurality of semiconductor chips is connected; a wideconductor to which a face on the other side of said plurality ofsemiconductor chips is connected; a second conductor connected to saidwide conductor; a cooler to which said first conductor and secondconductor are connected through an insulating resin sheet; (2) anelectric motor incorporating said inverter device and that drives adrive wheel by using an AC power from said inverter device; and (3) aninternal combustion engine that drives said drive wheel and is providedin addition to said electric motor.